When wood burns, three things happen:
- Water is removed by evaporation
- Chemically, the wood breaks down into charcoal, gas and volatile liquids, with carbon dioxide and water being the chief end products
- The charcoal burns, forming carbon dioxide either directly or with an intermediate conversion to carbon monoxide.
One pound of very dry (zero moisture content) wood of any species has a calorific value of approximately 8,600 Btu (British thermal unit, which equals the amount of heat required to raise the temperature of one pound of water one degree F). Any moisture in the wood reduces the recoverable heat by carrying heat up the chimney during vaporization. Each pound of water vaporized uses about 1,200 Btu.
Additional Btu are lost through the formation of volatile liquids and gases during combustion, but these vary by the type of heating unit and should be considered part of the efficiency factor of the heating unit.
A pound of wood with a 20-percent moisture content contains 0.17 pound of water and 0.83 pound of completely dry wood and has a heat value of about 7,000 Btu